Sidang video

George W. Bush mengadakan sidang video di Offutt Air Force Base

Sidang video (juga dikenali sebagai telepersidangan video atau telekonferens video, singkatan "VTC") merupakan satu set teknologi telekomunikasi interaktif yang membenarkan orang di dua atau lebih lokasi berinteraksi melalui penghantaran video dan audio dua-hala secara serentak. Ia juga telah dirujuk sebagai kerjasama pandang dan merupakan sejenis perisian kumpulan. Ia berbeza dari videofon kerana ia direka untuk ditujukan kepada suatu persidangan dan bukan individu.

Sidang video menggunakan telekomunikasi audio dan video untuk menghubungkan orang di beberapa tempat untuk bermesyuarat. Ini boleh semudah perbualan di antara dua orang dalam pejabat persendirian (titik-ke-titik) atau melibatkan beberapa tempat (multi-titik) dengan lebih seorang dalam bilik besar di setiap tempat. Selain penghantaran audio dan visual aktiviti mesyuarat, sidang video boleh digunakan untuk berkongsi dokumen, maklumat paparan komputer, dan papan putih.

Sejarah

Sidang video analog ringkas telah wujud hampir seawal penciptaan televisyen. Sistem sidang video ini terdiri dari dua sistem televisyen litar tertutup yang disambungkan menggunakan kabel. Semasa penerbangan angkasa lepas dikendalikan manusia pertama, NASA menggunakan dua sambungan frekuensi radio (UHF atau VHF), satu untuk setiap arah. Saluran TV sering menggunakan sidang video seperti ini apabila melaporkan dari lokasi jauh. Kemudian sambungan bergerak ke satelit menggunakan lori yang dilengkapi peralatan khas menjadi lebih umum.

Teknik ini bagaimanapun amat mahal, dan tidak boleh diterapkan dalam kegunaan biasa, seperti teleperubatan, pendidikan jarak jauh, mesyuarat perniagaan, dan seterusnya, terutamanya apa-apa kegunaan jarak jauh. Percubaan untuk menggunakan rangkaian telefoni biasa untuk menghantar video imbasan perlahan, seperti sistem pertama yang dibangunkan oleh AT&T, selalunya gagal disebabkan oleh kualiti gambar yang teruk dan ketiadaan teknik pemampatan video yang cekap. Lebar jalur 1 MHz dan kadar Picturephone 6 Mbit/s bit yang lebih besar pada 1970-an juga gagal menjayakan perkhidmatan ini.

Cuma pada 1980-an barulah rangkaian penghantaran telefoni digital mampu disediakan, contohnya ISDN, menjamin kadar bit minimum (lazimnya 128 kilobit/s) untuk penghantaran video dan audio termampat. Sistem khusus yang pertama, seperti yang dibuat oleh firma pelopor seperti PictureTel, mula muncul di pasaran apabila rangkaian ISDN mula berkembang di seluruh dunia. Sistem telepersidangan video sepanjang 1990-an berkembang dengan pantas dari peralatan dan perisian proprietari mahal serta keperluan rangkaian yang tertentu, kepada teknologi piawai yang mudah didapati oleh masyarakat umum pada harga yang munasabah. Akhirnya, pada 1990-an, sidang video berasaskan IP (Internet Protocol) mula boleh digunakan, dan teknologi pemampatan video yang lebih cekap dibangunkan, membolehkan sidang video berasaskan komputer meja, atau komputer peribadi (PC). Pada 1992 CU-SeeMe dibangunkan di Cornell oleh Tim Dorcey et al., IVS pula direka di INRIA, membolehkan VTC digunakan ramai. Kini, perisian tertentu atau plugin web, sestengahnya boleh didapati percuma, seperti NetMeeting, MSN Messenger, Yahoo Messenger, SightSpeed dan Skype menyediakan perkhidmatan murah persidangan video, walaupun dengan kualiti agak rendah, kepada semua.

Teknologi

The core technology used in a videoteleconference (VTC) system is digital compression of audio and video streams in real time. The hardware or software that performs compression is called a codec (coder/decoder). Compression rates of up to 1:500 can be achieved. The resulting digital stream of 1s and 0s is subdivided into labelled packets, which are then transmitted through a digital network of some kind (usually ISDN or IP). The use of audio modems in the transmission line allow for the use of POTS, or the Plain Old Telephone System, in some low-speed applications, such as videotelephony, because they convert the digital pulses to/from analog waves in the audio spectrum range.

The other components required for a VTC system include:

• Video input : video camera or webcam
• Video output: computer monitor , television or projector
• Audio input: microphones
• Audio output: usually loudspeakers associated with the display device or telephone
• Data transfer: analog or digital telephone network, LAN or Internet

There are basically two kinds of VTC systems:

1. Dedicated systems (manufactured by companies such as Sony, Tandberg, Polycom, and Radvision Ltd.) have all required components packaged into a single piece of equipment, usually a console with a high quality remote controlled video camera. These cameras can be controlled at a distance to pan left and right, tilt up and down, and zoom. They became known as PTZ cameras. The console contains all electrical interfaces, the control computer, and the software or hardware-based codec. Omnidirectional microphones are connected to the console, as well as a TV monitor with loudspeakers and/or a video projector. There are several types of dedicated VTC devices:
   1. Large group VTC are non-portable, large, more expensive devices used for large rooms and auditoriums.
   2. Small group VTC are non-portable or portable, smaller, less expensive devices used for small meeting rooms.
   3. Individual VTC are usually portable devices, meant for single users, have fixed cameras, microphones and loudspeakers integrated into the console.
2. Desktop systems are add-ons (hardware boards, usually) to normal PCs, transforming them into VTC devices. A range of different cameras and microphones can be used with the board, which contains the necessary codec and transmission interfaces. Most of the desktops systems work with the H.323 standard. Videoconferences carried out via dispersed PCs are also known as e-meetings.

Echo cancellation

A fundamental feature of professional VTC systems is acoustic echo cancellation (AEC). AEC is an algorithm which is able to detect when sounds or utterances reenter the audio input of the VTC codec, which came from the audio output of the same system, after some time delay. If unchecked, this can lead to several problems including 1) the remote party hearing their own voice coming back at them (usually significantly delayed) 2) strong reverberation, rendering the voice channel useless as it becomes hard to understand and 3) howling created by feedback. Echo cancellation is a processor-intensive task that usually works over a narrow range of sound delays.

Multipoint videoconferencing

Simultaneous videoconferencing among three or more remote points is possible by means of a Multipoint Control Unit (MCU). This is a bridge that interconnects calls from several sources (in a similar way to the audio conference call). All parties call the MCU unit, or the MCU unit can also call the parties which are going to participate, in sequence. There are MCU bridges for IP and ISDN-based videoconferencing. There are MCUs which are pure software, and others which are a combination of hardware and software. An MCU is characterised according to the number of simultaneous calls it can handle, its ability to conduct transposing of data rates and protocols, and features such as Continuous Presence, in which multiple parties can be seen onscreen at once.

MCUs can be stand-alone hardware devices, or they can be embedded into dedicated VTC units.

Some systems are capable of multipoint conferencing with no MCU, stand-alone, embedded or otherwise. These use a standards-based H.323 technique known as "decentralized multipoint", where each station in a multipoint call exchanges video and audio directly with the other stations with no central "manager" or other bottleneck. The advantages of this technique are that the video and audio will generally be of higher quality because they don't have to be relayed through a central point. Also, users can make ad-hoc multipoint calls without any concern for the availability or control of an MCU. This added convenience and quality comes at the expense of some increased network bandwidth, because every station must transmit to every other station directly.

Isu

Some observers argue that two outstanding issues are preventing videoconferencing from becoming a standard form of communication, despite the ubiquity of videoconferencing-capable systems. These issues are:

1. Eye Contact: It is known that eye contact plays a large role in conversational turn-taking, perceived attention and intent, and other aspects of group communication. While traditional telephone conversations give no eye contact cues, videoconferencing systems are arguably worse in that they provide an incorrect impression that the remote interlocutor is avoiding eye contact. This issue is being addressed through research that generates a synthetic image with eye contact using stereo reconstruction.
2. Appearance Consciousness: A second problem with videoconferencing is that one is literally on camera, with the video stream possibly even being recorded. The burden of presenting an acceptable on-screen appearance is not present in audio-only communication. Early studies by Alphonse Chapanis found that the addition of video actually impaired communication, possibly because of the consciousness of being on camera.

The issue of eye-contact may be solved with advancing technology, and presumably the issue of appearance consciousness will fade as people become accustomed to videoconferencing.

Piawai

The International Telecommunications Union (ITU) (formerly: Consultative Committee on International Telegraphy and Telephony (CCITT)) has three umbrellas of standards for VTC.

1. ITU H.320 is known as the standard for public switched telephone networks (PSTN) or VTC over integrated services digital networks (ISDN) basic rate interface (BRI) or primary rate interface (PRI). H.320 is also used on dedicated networks such as T1 and satellite-based networks;
2. ITU H.323 is known as a standard for transporting multimedia applications over LANs. This same standard also applies to older implementations of voice over IP VoIP. In recent years, the IETF's Session Initiation Protocol (SIP) has gained considerable momentum in practice for these two services.;
3. ITU H.324 is the standard for transmission over POTS, or audio telephony networks. 3G-324M is a 3GPP implementation for video call on 3G mobile phones.

In recent years, IP based videoconferencing has emerged as a common communications interface and standard provided by VTC manufacturers in their traditional ISDN-based systems. Business, government and military organizations still predominantly use H.320 and ISDN VTC. Though, due to the price point and proliferation of the Internet, and broadband in particular, there has been a strong spurt of growth and use of H.323, IP VTC. H.323 has the advantage that it is accessible to anyone with a high speed Internet connection, such as DSL.

In addition, an attractive factor for IP VTC is that it is easier to set-up for use with a live VTC call along with web conferencing for use in data collaboration. These combined technologies enable users to have a much richer multimedia environment for live meetings, collaboration and presentations.

Lihat juga

• Telepresence
• Telesidang
• Mobile VoIP
• Telefon video untuk panggilan peribadi

Pautan luar

• ViDe Videoconferencing Cookbook
• Videoconferencing businesses di Curlie